By Tom Skilling
It's been four years since Chicago has been placed under an "excessive heat warning" by the National Weather Service. The current warning runs through 7 p.m. Friday as do the heat advisories which have been hoisted over the remainder of the Chicago area.
Guidelines for the issuance of such advisories were developed in the wake of the city's deadly July 1995 heat wave. To this day -- 15 years later -- the event, which claimed more than 700 lives, remains Chicago's single worst natural disaster. In that hot spell, temperatures soared to 106 degrees at Midway Airport which, in tandem with stifling atmospheric moisture levels that included dew points of 82 degrees, led to a never-before-seen 125-degree heat index here.
The elderly and those with pre-existing medical conditions aggravated by the onset of heat comprised the majority of the deadly heat wave's victims. But the extent of death at the time took area residents and officials by surprise and led to a major effort to more clearly understand the combination of atmospheric conditions which were capable of generating such human carnage.
The current hot spell, which is expected to ease late this weekend and early next week, isn't anywhere near as extreme. Neither temperatures nor humidities will come close to 1995's off-the-charts levels. But the low to mid 90s predicted Thursday and Friday coupled with oppressive afternoon dew points at or above 75 degrees will lead to a 98 to 107-degree peak heat indices.
Excessive heat warnings are issued when any of the following three conditions are predicted: three consecutive days of heat indices 100 to 105 degrees; two consecutive days with heat indices 105 to 110, or a single day in which the critical index tops 110-degrees.
Deaths from heat top those which occur in all other weather extremes
Far from being just a nuisance, heat is the most deadly of all extreme weather events. The number who perish annually across the U.S. as a result of extreme heat exceeds those killed by tornadoes, flash floods, hurricanes, lightning or cold weather.
There are now estimates that as many as 15,000 may have already died in Russia in the record heat wave which is predicted to continue there for the foreseeable future after a brief break Wednesday. And the final tally could end up even worse. Summer temperatures have reached 100 degrees at Moscow weather stations in the past several months, the first time in 130 years of weather records there that triple-digit reading have occurred.
Temperatures in Moscow for the meteorological summer season which began June 1 have topped 90 degrees on 31 days and have reached or exceeded 100 degrees five days -- a stunning development in a city with "normal" summer highs in the low to mid 70s. The summer season there has averaged a remarkable 9.6 degrees above normal. All are stats which place the modest nature of Chicago's latest flirtation with heat and humidity in perspective.
Heat Thursday extends 80-degree streak to 42 days -- the most here in 55 years
Wednesday's 90-degree high at O'Hare became the official site's 16th day at or above 90 degrees while Midway's 91-degree reading marked the South Side site's 18th such day. Not even the day's lake breeze was able to spare the city's lakefront from a rendezvous with heat. Northerly Island topped out at 93.
Fewer thunderstorms than in many recent weeks here are to provide a backdrop for the hot temperatures predicted Thursday and Friday. Thermometer readings are predicted to top out at 94 Thursday and 95 Friday. Each reading would tie or exceed 2010's highest temperature to date: 94 degrees at O'Hare and 95 degrees at Midway and the lakefront recorded back on July 23. It also assures the city's record for consecutive 80 degree or higher temperatures based on over 140 years of official weather records will occur Thursday and that a new record will be established Friday.
St. Louis really hot; a 50th day over 90 due Thursday
As hot as Chicago's weather will be the next few days, residents of St. Louis -- where the temperature hit 100 degrees Wednesday and the heat index topped out at 112 degrees -- are to mark 2010's 50th day at or above 90 degrees Thursday. There could be another 100-degree reading today.
Seventeen states remain under heat advisories or excessive heat warnings the remainder of the work week.
Huge pattern change has an early autumnal look next week
Big changes in North America's weather pattern loom. The dome of warm air so frequently a part of the meteorological scene over Greenland and the northwest Atlantic in recent years -- the feature referenced so often here as the "Greenland Block" -- gets back in business in the coming week. It's predicted by computer models that it will produce strong ridging over the western Atlantic, which induces strong troughing aloft over central North America. The northwest steering winds that will run from Canada into the nation's heartland will arrive late this weekend and into next week. They have an early autumn look and promise to drop temperatures and humidities here.
But it's unlikely we've seen the last of this year's hot weather. Warming is predicted again the following weekend.
